1. Field of the Invention
The present invention relates to an apparatus for manufacturing an optical disc which has a transfer layer carrying fine undulations of a liquid radiation setting resin (which will be shortly referred to as a "photopolymer"). The resin is set by exposure to radiation such as ultraviolet rays. The optical disc used herein means a substrate one which a recording layer will be applied in a subsequent process as is well known in the art.
2. Description of the Prior Art
In the prior art, two optical discs or substrate have been employed, one of which is formed on a transparent disc and has a row of pits representing information signals arranged in concentric rings or a spiral shape, and a second of which has a spiral guide groove into which additional information can be written using a laser spot, after the disc has been manufactured. The process for manufacturing an optical disc having fine undulations on its surfaces such as the pit row or a guide groove is described in Japanese Patent Laid-Open Nos. 53-116105, 54-23501 and 54-2350, for example. In any of these manufacturing processes, the photopolymer is carried on the disc and the transfer region of a stamper having predetermined fine undulations (or patterns) thereon is brought into contact with the photopolymer. This photopolymer is then exposed to ultraviolet rays causing it to set and thus form a transfer layer on the disc.
However, when the photopolymer is extended between the disc and the stamper, according to these manufacturing processes, a defect occurs, wherein fine foams are created in the photopolymer. Generally speaking, the pitch of the pit rows in the radial direction of the optical disc is as small as about 1.6 microns. As a result, the creation of foams in the photopolymer during the transfer step is detrimental to the drop-out of the pits or the guide groove.
A technique for preventing the formation foams in the photopolymer between the disc and the stamper is disclosed in our Japanese Patent Laid-Open No. 60-261047 which is directed to an optical disc manufacturing process. This manufacturing method will be described with reference to FIG. 16 showing a portion of an optical disc manufacturing apparatus.
Initially, as shown in FIG. 16(a), a stamper 1 is fixed in position on a base 2 and supplied with a photopolymer 5. Then, a transparent disc 4 made of PMMA is placed on the outer circumferential flange of a center boss 3.
Next, the center boss 3 is dropped to bring the disc 4 into contact with the photopolymer on the stamper 1. In this state, foams are confined in a photopolymer 5.
Next, as shown in FIG. 16(b), the disc 4 is held at its outer circumference by a holding member 6 (or ring) such that the outer circumferences of the stamper 1 and the disc 4 sandwich the photopolymer 5 therebetween.
Next, as shown in FIG. 16(c), the center boss 3 is moved upwards to push the inner circumference of the disc 4 upward. Since the disc 4 has its outer circumference fixed in place by the outer circumferential ring 6, the disc 4 is deformed into a conical or Konide shape, as shown. Thus, an enclosed space having a flattened conical shape is formed over the center recess of the base 2 and between the disc 4 and the stamper 1. As the center boss 3 moves upward, the photopolymer 5 spreads toward the outer circumference. At this time the larger foams having a diameter of 0.5 mm or more in the photopolymer 5 disappear in accordance with the outward spread of the photopolymer. However, the smaller foams are collected and confined in the annular stagnant photopolymer.
Here, the closed space formed among the center boss 3, the disc 4 and the stamper 1 communicates with a gas pressure control means for pressurizing or evacuating the area in the vicinity of the center boss. As a result, the innermost circumference of the photopolymer 5 vibrates, thus eliminating the foams in the photopolymer 5 when a small amount of pressurized gas is supplied from the center recess of the base 2 by the gas pressure control means while a sealed state exists.
Next, during a transfer step as shown in FIG. 16(d), the pressurization is interrupted, while the sealed state of the center recess is maintained, and the gas is sucked out by evacuation. Then, the photopolymer 5 flows from the outer circumference to the inner circumference between the stamper 1 and the disc 4 as a result of the pressure difference from the atmospheric pressure. The center boss 3 is then dropped. This causes the photopolymer 5 flowing from the outer to the inner circumference between the disc 4 and the stamper 1 to reach the innermost circumference of the transfer region until it is either completely consumed before it reaches the innermost circumference of the stamper 1 or is discharged to the outermost circumference of the stamper 1.
Next, as shown in FIG. 16(e), the center boss 3 is further dropped until it stops at a predetermined position where the entire surface of the disc 4 is generally coextensive with the stamper surface.
The outer circumferential ring 6 is then removed while the evacuation of the enclosed space in the vicinity of the lower side of the inner circumference of the center boss 3 is maintained, and the photopolymer 5 is set by exposure to ultraviolet rays.
If the disc 4 is separated from the stamper 1 by lifting the center boss 3 after the photopolymer 5 has been set, it is possible to obtain a disc 4 which has a set photopolymer layer 5 onto which the fine undulations of the stamper 1 have been transferred.
In this manufacturing process, however, if the evacuation is accomplished while the sealed state is maintained in the center recess, the photopolymer 5 is caused to proceed from the outer circumference to the inner circumference between the stamper 1 and the disc 4 as a result of the pressure difference from atmospheric pressure, as shown in FIG. 16(d). When the photopolymer 5 flows from the outer to the inner circumferences it may confine fine foams, which might not be eliminated by the cooperation between the outer circumferential ring and the center boss. The confinement of the fine foams in the photopolymer 5 lowers the yield of the optical disc.